(1) Field of the Invention
This invention relates to hydrophilized porous membranes useful in such fields as water treatment and blood purification and to their production process, and more specifically to porous polyolefin membranes with their pores covered by a hydrophilic polymer and to a production process thereof.
(2) Description of the Prior Art
The fields of application of porous polyolefin membranes are expanding rapidly due to their excellent mechanical properties and chemical resistance. Porous polyolefin membranes are however hydrophobic and when used as is, water is allowed to permeate therethrough with difficulty. A hydrophilizing treatment is therefore indispensable to have hydrophilic liquids including water permeate therethrough. A variety of methods have been studied with a view toward imparting hydrophilicity through surface modification of polyolefin membranes. Hydrophilizing methods, which have been proposed for film-like materials featuring smooth surfaces, cannot be simply applied to impart hydrophilicity to porous membranes having complex surface configurations.
As hydrophilizing methods for porous polyolefin membranes, there have been known the organic solvent wetting and water substituting method in which the entire surface of a porous polyolefin membrane, inclusive of minute pores, is subjected to a wetting treatment with an organic solvent having good miscibility with water such as an alcohol or ketone, followed by substitution of water for the organic solvent; the physical adsorption method in which a hydrophilic material such as polyethylene glycol or a surfactant is adsorbed on a surface of a porous membrane so as to impart hydrophilicity to the porous membrane (Japanese Patent Laid-Open Nos. 153872/1979 and 24732/1984); and the chemical surface modification method in which a porous membrane is exposed to radiation while holding a hydrophilic monomer on a surface of the membrane (Japanese Patent Laid-Open No. 38333/1981) or the porous structure of a hydrophobic resin is subjected to a plasma treatment in a state impregnated with a water-soluble high-molecular material and a surfactant (Japanese Patent Laid-Open No. 157437/1981).
In the organic solvent wetting and water substituting method, if water is once lost from minute pores during storage or use, the part containing these water-free minute pores regains hydrophobicity and no longer permits the permeation of water therethrough. Accordingly, since it is always necessary to keep water around the porous membrane, the porous membrane is difficult to handle. Although the physical adsorption method is easy to practice, the hydrophilic material drops off if the resulting porous membrane is used over a long period of time. Therefore, this method cannot be regarded as a fully satisfactory hydrophilizing method. The conventional chemical surface modification method is accompanied by one or more problems. It is difficult to impart uniform hydrophilicity in the direction of the thickness of a membrane, when the porous membrane is exposed to radiation or subjected to the plasma treatment. If one attempts to apply a hydrophilizing treatment uniformly over the entire thickness of a porous membrane when the membrane has a large thickness or is in the form of a hollow fiber, the mechanical strength of the matrix of the porous membrane is unavoidably reduced, leading to damage.